Method of vaporizing metal

ABSTRACT

A method of vaporizing metal which comprises the steps of forming a thin shell of molten metal the metal of which is above and communicated with a supply of molten metal, heating said thin shell of molten metal to vaporize said metal, and withdrawing the vapor from the shell and transporting the vapor to a point at which the vapor will impinge upon a substrate, by means of path which removes any molten metal particles from the metal vapor.

States atent [1 1 Rohlin et a1,

[ Sept. 17, 1974 METHOD OF VAPORIZING METAL [75] Inventors: John M. Roblin, Cleveland; Frank J.

Cole, Parma; William A. Reed, Richfield, all of Ohio [73] Assignee: Republic Steel Corporation,

Cleveland, Ohio 22 Filed: Dec.29,11971 21 Appl. No.: 213,764

Related US. Application Data [62] Division of Ser. No. 590,921, Oct. 31, 1966, Pat. No.

[52] US. Cl 1117/1112 A, 117/107, 117/107.1 [51] int. Cl C23c 13/02 [58] Field of Search 117/107, 107.2,.102 M,

[56] References Cited UNITED STATES PATENTS 1,265,863 5/1918 Abbott 118/48 2,446,557 8/1948 Schutz et al. 118/48 2,664,852 l/l954 Chadsey 118/49 3,059,612 10/1962 Baughman et a1. ll8/49.1

Primary Examiner-Charles E. Van Horn Assistant Examiner-4. Massie Attorney, Agent, or Firm-Cooper, Dunham, Clark, Griffin & Moran [5 7] ABSTRACT A method of vaporizing metal which comprises the steps of forming a thin shell of molten metal the metal of which is above and communicated with a supply of molten metal, heating said thin shell of molten metal to vaporize said metal, and withdrawing the vapor from the shell and transporting the vapor to a point at which the vapor will impinge upon a substrate, by means of path which removes any molten metal particles from the metal vapor.

4 Claims, 4 Drawing Figures PAIENILBSEWW 3836.387

SHEEI 1 BF 3 V'ACUUM PUMP METHOD OF VAPORIZING METAL This application is a division of my co-pending Application Ser. No. 590,921, filed Oct. 31, 1966, now U.S. Pat. No. 3,672,327, for VAPORIZATION OF METAL FOR VACUUM METALIZING.

This invention relates to evaporating techniques. More particularly, it relates to the vaporization of metal for vacuum deposition.

In the past, vacuum deposition of metal has been accomplished by the boiling of the metal to be vaporized in an open pot or boat. Disadvantages with such a technique are irregular or violent boiling, and especially the formation of bubbles which grow to large size before they reach the surface and burst. All of this leads to splattering as well as excessive entrainment of liquid particles in the metal vapor. Liquid entrainment in the metal vapor is undesirable inasmuch as it adversely affects the quality of the metal coating obtained by the deposition of the metal vapor. At the same time, in vacuum deposition of a metal such as zinc where it may be desired to provide operation on a considerable scale as an alternative to the common procedure of hot-dip galvanizing, evaporating means and methods are needed which can produce relatively large quantities of metal vapor at constant rate with a readily available heat source such as one utilizing combustion of oil, gas or other fuel, or electrical heating units, e.g., of heatradiating type.

With constant heat input supplied for vaporization, the rate of vaporization of metal tends to vary with the area in contact with the heat source of the metal available for vaporization. Hence in any batch process such as that employing a boat, as the metal in the boat is va porized the contact area decreases, and the rate of vaporization consequently decreases. It is desirable, however, to maintain the rate of vaporization constant in any vacuum deposition process.

Accordingly, an object of the present invention is to provide improved vaporizing methods.

A further object of the present invention is to provide for the vaporization of metal while avoiding excessive boiling as well as liquid entrainment.

Still another object of the present invention is to provide for the vaporization of metal wherein the rate of vaporization is constant regardless of the quantity of molten metal available for vaporization.

An additional object is to afford improved and economical procedure for evaporation of zinc to yield a substantially constant and desirably voluminous flow of zinc vapor.

These and other objects of the present invention are achieved through the use of vaporizing apparatus in which two containers, such as inner and outer concentric vessels, are employed which communicate with each other and which contain metal to be vaporized. Heating means is provided to directly heat one of the containers so that the body of metal therein boils before the body of metal in the other container can boil. The rate of application of heat to the heated container is such as to provide for the continued boiling of the body of metal therein but not of the body of metal in the other container, which remains molten but substantially nonboiling. As the metal in the heated container boils, its density decreases because of the presence of vapor bubbles therein, and it is much less dense than the substantially nonboiling metal in the other container. Because of the communication of the containers, the pressure of the substantially nonboiling body of metal causes the boiling body of metal to rise in its container until the pressures of the two bodies of metal are equal. In practice, the heated container tends to be completely filled with molten, boiling metal for all levels of liquid in the other container except extremely low levels. Because the heated container is always filled with metal which is being vaporized, the effective quantity of metal for vaporization is constant regardless of the overall supply of metal in the entire batch, i.e., the area of contact between heat source and metal remains constant. Hence the rate of vaporization does not vary with the total amount of metal within the containers.

Outlet means is provided from the heated container to provide an outlet for the vapor produced. Such outlet means may include baffle means for removing droplets of liquid within the vapor which, if allowed to remain entrained in the vapor, would render the deposited coating nonuniform.

When inner and outer concentric vessels are employed for the containers, the space between the inner and outer vessels may serve as the volume for vaporization while the space within the inner vessel comprises the volume for the substantially nonboiling molten metal. The heating means may comprise pipes positioned outside of the outer vessel which carry a fluid heat exchange medium therein to transfer heat to the outer vessel and thus to the volume of metal to be vaporized. Electrical heating units of heat radiating type may also be employed, or the outer vessel may be enclosed by a shell which is directly heated by flame, e.g., so as to provide for the radiant heating of the outer vessel. The vessels advantageously communicate with each other solely at the top and bottom portions thereof, and an outlet pipe is provided which is positioned within the inner vessel and below the level at which the inner and outer vessels communicate with each other at their top portions. The vapor from the outer body of metal, then, is caused to follow a tortuous path, namely, from the top of the space between the outer and inner vessels to the inner vessel, downwardly to the opening of the outlet pipe and thence upwardly through the outlet pipe ultimately to a deposition nozzle. This arrangement of outlet pipe provides an effective baffle, as described above, to remove liquid particles which are entrained within the vapor and to return them to the molten body of metal in the inner vessel. Futher, the space between the vessels provides a relatively thin shell of fluid which presents a relatively large area for the application of heat thereto and which reduces somewhat the size of bubbles that are formed. The bubbles have a high velocity, however, and hence the average heat transfer coefficient is satisfactory.

The containers of molten metal, both inside and outside thereof, and the deposition nozzle are all maintained under vacuum to aid in the deposition process. Since the containers are not subjected to a large pressure difference between the outside and the inside thereof, elaborate sealing arrangements need not be employed.

The invention will be more completely understood by reference to the following detailed description. In the accompanying drawings:

FIG. 1 is a diagrammatic representation of vacuum deposition apparatus for carrying out the invention.

FIG. 2 is a sectional view to an enlarged scale of a representative form of the vaporizing portion of the apparatus shown in FIG. 1.

FIG. 3 is a sectional view of the vaporizing apparatus shown in FIG. 2, taken along the section line 3-3 and looking in the direction of the arrows in FIG. 3.

FIG. 4 is a sectional view of an alternative form of vaporizing portion of the apparatus.

Referring to FIG. 1, a strip of metal to be coated by vapor deposition is shown conveyed from a supply reel 12 to a take-up reel 14. The strip of metal passes by a deposition nozzle 16 from which vapor of coating metal issues. The nozzle is supplied with vapor from a vaporizing unit 18. A heat exchange medium, such as hot combustion gases, is applied to the vaporizing unit 18 via an inlet 20 and is removed from the unit by an outlet 22 to provide the necessary heat for the vaporization of coating metal for deposition. The vaporizing unit 18 includes an outlet section 24 which is coupled to a deposition chamber 26 enclosing the deposition nozzle 16 as well as the strip 10 and supply and take-up reels 12 and 14. The interiors of the chamber 26 and the vaporizing unit 18 are maintained under vacuum during the deposition process by a vacuum pump 27.

The illustration of chamber 26 and its contents is purely diagrammatic, omitting mechanical details, which may be conventional, as for supporting and operating the reels; in practice, the actual shape, size and arrangement of the chamber will be such as is appropriate for coating operation on a desired strip or other article.

FIG. 2 shows the details of the vaporizing unit 18. The unit comprises an enclosure 28 typically of steel which is lined with an insulating refractory material 30, e.g., an aluminum silicate ceramic fiber such as Kaowool" manufactured by Babcock & Wilcox. The enclosure 28 is advantageously cylindrical, as shown in FIG. 3, and includes as its upper portion the outlet section 24 leading to the deposition nozzle 16 of FIG. 1. The top of the container is closed by a cover 32 typically of steel. The cover may also be lined with an insulating refractory material 34. An annular rim or flange 36 surrounds the enclosure 28 and aids in supporting the cover. The refractory material lining the interior of the container 28 is in turn lined with a radiation shield 38 which typically may be of stainless steel. Similarly, the refractory material 34 lining the cover 32 may be in turn lined with a radiation shield 40 also of stainless steel.

Disposed inside the enclosure 28 is a tubular crucible 42 which may be made of heat conducting composition that can be deemed refractory for present purposes, such as silicon carbide or graphite, e.g. The crucible includes a base portion 42a which is seated upon refractory material 43, such as brick, which lines bottom plate 28a of the enclosure. A tubular liner 44 is positioned within the crucible 42, the bottom of which is seated upon bottom 42b of the crucible. The liner and crucible respectively comprise inner and outer vessels which communicate with each other through openings 44a in the bottom of the liner and openings 44b and 44c in the top of the liner. The liner extends upwardly and abuts against ledge 46a of a support member 46 which is advantageously formed of graphite. The support member 46 may be made of an aluminum silicate bonded, high alumina castable refractory or graphite e.g. The support member includes at its upper portion an outwardly extending rim 46b which rests upon the upper end of the crucible 42. The support member 46 includes passages 46c and 46d therein. A baffle pipe 48 advantageously of alumina is positioned within the passage 46d and is frictionally or otherwise secured to the support member 46.

A crucible cover 50 typically of graphite is positioned against the end of the support member 46. Alternatively, a cover made of a glass sealing alloy formed of nickel, cobalt, manganese and iron, e.g., may be employed, if desired. The cover is generally tubular and its lower end fits into the passage 46c. An annular rim 50a forming a part of the cover rests against the upper end of the support member 46 to maintain the cover in position. An outlet pipe 52 extends from the cover and is secured by coupling 54 to anotheroutlet pipe 56 that extends through the outlet section 24 of the vaporizing unit. A tube 58 is supported by the crucible cover 50 and extends through the passage 46c and through the baffle pipe 48 and through the liner 44 to a point adjacent the lower end of the liner. Tube 58 may contain a thermocouple to be used for the measurement of the temperature of molten liquid in the liner.

A plurality of pipes 60 surrounds the crucible 42. These pipes may be U-shaped, e.g., with one end thereof connected to a manifold 62 and the other end thereof connected to a manifold 64, as shown in FIGS. 2 and 3. As shown in FIG. 3, the manifold 62 is connected to the inlet pipe 20 which receives a hot combustion gas, e.g., while the manifold 64 is connected to the outlet pipe 22 to discharge the hot combustion gas. The hot combustion gas supplied to the pipes 60 surrounding the crucible 42 serves to heat the crucible for the evaporation of metal, as described more completely below.

In operation, a metal to be vaporized is supplied in a batch quantity to the crucible 42. For this purpose the main enclosure cover 32 is removed. The cover 50 may be lifted for the insertion of metal into the crucible by removing the coupling 54, sliding the outlet pipe 56 to the right with respect to FIG. 2, and lifting the crucible cover 50 sufficiently so as to pour molten metal or granular solid metal into the passage 46c. Thereafter, the covers 50 and 32 are replaced. The vaporizing unit 18 as well as the deposition chamber 26 of FIG. 1 are maintained under proper vacuum by the vacuum pump 27 for vapor deposition of metal. Hot fluid such as burnt combustion gases is circulated through the pipes 60 to maintain the metal within the crucible 42 and within the liner 44 in a molten state. The metal circulates from space within the liner 44 to space 72 between the liner and the crucible 42 through the circulation holes 44a at the bottom of the liner. Initially, the metal in the space 72 assumes a level the same as the level of the metal in the space 70. Heat from the pipes 60 is reflected inwardly by the radiation shields 38 and 40 through the crucible 42, through the metal in the space 72, through the liner 44 and thence into the molten metal within the liner. Hence the molten metal within the space 72 between the liner and the crucible (outer volume of metal) is heated initially and to a higher temperature than that of the body of molten metal in the space 70 within the liner (inner volume of metal). This outer volume of metal reaches its boiling point before the inner body of molten metal. As the outer body of metal boils, vapor bubbles are formed within that body and thus lower the density of the metal. The level of boiling metal in the space 72 is raised to a higher level than that of the nonboiling main body of metal in the space 70 because of the difference in density of the two bodies of metal. The outer body of boiling metal completely fills the space 72 between the crucible and the liner at least to a level below the circulation holes Mb and Me. The difference in density between the two bodies of metal is such that the boiling metal between the crucible and liner is always to the level of the circulation holes Mb and Me, regardless of the level of the main body of molten metal within the liner 44, except for very low levels of molten metal in the liner. Hence a constant volume of boiling metal is maintained in the space 72 between the crucible and liner, i.e., constant contact area between metal and heat source is maintained, which permits a constant vaporization rate to take place. In other words, percolation of the outer volume of fluid in the space 72 takes place and causes continuous circulation of fluid between the outer and inner volumes.

Vapor and molten metal from the boiling body of metal in the space 72 between the crucible and the liner pass through the circulation holes 44b and Me and downwardly within the liner M. Droplets of molten metal continue downwardly into the body of metal within the liner, while vapor passes upwardly into the lower opening of the baffle pipe 48 and continues upwardly through the pipe and ultimately through the outlet pipe 56. The vapor thus undergoes a tortuous path provided by the baffle pipe which removes from the vapor any particles of liquid that may be entrained therein. In particular, such particles pass into the body of molten metal within the liner 44 while the vapor passes upwardly through the pipe 48. Removal ofliquid droplets from the vapor is of decided advantage in the vapor deposition of metal for coating purposes, inasmuch as the coating is made much more uniform.

The temperature of the body of molten metal within the liner M is maintained just below the boiling point, so that the body of metal in the space 72 between the liner and the crucible is maintained at the boiling point. The thermocouple may be used to monitor the temperature of the fluid within the liner 44.

The concentrically positioned crucible 42 and liner 44 provide two bodies of fluid. The outer body of fluid in the volume 72 between crucible and liner is for the boiling of metal while the inner body of fluid in the volume 70 within the liner 44 is for the storage of substantially nonboiling molten metal. The volumes of the regions 7t) and 72 may be roughly the same. However, because of the concentricity, a relatively thin shell of fluid is provided within the region 72. Such a thin shell of fluid has two advantages. First, the thinness of the shell reduces the size of bubbles of vapor that are formed. Second, by forming a relatively thin shell of fluid, a relatively large surface area is provided for the application of heat to that volume of fluid, which increases the efficiency of the transfer of heat for vaporization. The efficiency of heat transfer is also increased by the high velocity of bubble movement through the thin shell of fluid. The inner and outer concentric volumes of fluid further facilitate the heating of only one of those volumes of fluid for boiling, as described above. In this regard, the rate of application of heat through the crucible 42 is chosen so that almost all of the heat supplied constitutes the heat of vaporization of the fluid in the region '72 between the crucible and liner. Hence the fluid within this region boils while the molten fluid in the region remains substantially nonboiling.

In a test that was performed employing apparatus, e.g., as shown in FIGS. 1-3, molten zinc was added to the crucible 42. The batch of zinc initially occupied approximately 50 percent of the available volume in the crucible. Burnt gases at temperatures up to 2,070F. were circulated through the heating tubes 60 to heat the metal within the crucible and liner. The molten body of metal within the liner 44 was at a temperature of l,300F. A strip of steel having a thickness of0.0l 5 inch and a width of 6 inches was passed by the deposition nozzle 16 at controlled rates up to 63 feet per minute and was spaced from the nozzle 1 inch. The vacuum within the chamber 26 and the vaporization unit 18 was maintained between 1 and 10 microns of mercury. With this arrangment, the maximum evaporation rate of metal was approximately 0.4 pounds per minute and metal was deposited on the strip at: up to 0.3 pounds per minute. The coating that was obtained was uniform and from 400 to 1120 X 100' inches thick, depending on strip speed.

FIG. 4 shows a modified form of vaporizing apparatus similar to the vaporizing apparatus shown in FIGS. 2 and 3. Components in FIG. 4 which correspond to components in FIGS. 2 and 3 are given the same reference numerals as employed in FIGS. 2 and 3, with the addition of primes to the numerals of FIG. 4. In FIG. 4 the vaporizing apparatus includes an enclosure or receptacle means of refractory material 80a, such as an aluminum silicateceramic fiber, which may be covered by an outer layer of insulation 80b. A shell 82, typically of high temperature stainless steel, is positioned within the enclosure 80 and rests upon base portion 80c of the enclosure. The shell is heated by a flame which is directed against the shell by nozzles 84 included in the enclosure 80.

Positioned inside of the shell 82 is a crucible 42, typically of graphite or silicon carbide, which in turn encloses a liner Ml typically of graphite. Bottom portion 4l2a' of the crucible rests upon a support member of refractory material, e.g., aluminum oxide. Space 72' comprises an outer volume for the boiling of molten metal, while space 70 comprises an inner volume for molten metal which is substantially nonboiling. Passages Ma' are provided through the liner in the bottom portion thereof, and passages 44b and 440' are provided through the liner in the top portion thereof to provide communication between the inner and outer volumes of fluid as described above with respect to the embodiment of FIGS. 2 and 3. It will be noted in FIG. 4 that passages Mb and Me are inclined upwardly from the insideof the linerdd to the outside thereof, whereas in FIG. 2 the corresponding passages are illustrated as being horizontal.

As the flame from the nozzles 84 heats the shell 82, heat is transferred by radiation from the shell to the crucible 42 for boiling of the outer volume of metal in the space 72' as noted above. Vapor and molten metal pass from the passages 44b and 44c into passage 460' defined by support member 46, which may be of carbon or refractory material. Support member 46' includes a downwardly depending portion 46-l which corresponds to the baffle pipe 48 in the embodiment of FIGS. 2 and 3. The bottom of the downwardly depending portion dti-I is positioned below the level of the passages 44b and 44c, thus causing the vapor to follow a tortuous path in its course ultimately through the passage 46c and outwardly through outlet pipe 56' Metal to be vaporized is typically supplied to the crucible 42 in batch quantity by removal of covers 50 and 32 (which includes radiation shield or insulation 34). Following the application of the metal to be vaporized, covers 32' and 50' are replaced and the vaporizing process is initiated. It should be noted that the outlet pipe 56' is surrounded in a portion thereof by heating elements 86 which heat the outlet pipe and aid in maintaining the vapor at the desired vaporization temperature.

As an example only, some representative dimensions of vaporizing apparatus constructed in accordance with the embodiment shown in FIG. 4 are given as follows: The enclosure 80 was square in cross section, approximately 48 inches on a side measured along an outer surface. The shell 82, crucible 42 and liner 44 were cylindrical in shape with diameters of approximately 28 inches (outer diameter), 23% inches (outer diameter) and 15 inches (inner diameter), respectively. The heights of the crucible 42', liner 44 and shell 82 were approximately 70 inches, 60 inches and 106 inches, respectively. The diameter of the outlet pipe 56 was roughly 5 inches (inside diameter) in the region adjacent the passage 460.

In a test that was performed employing apparatus as shown in FIG. 4, molten zinc was applied to the crucible 42' to be vaporized. The batch of zinc initially occupied approximately two-thirds of the available volume in the crucible. Burnt gases at a temperature of l,800F. were circulated outside the shell 82 to heat the metal within the crucible and liner. The molten body of metal within the liner 44' was maintained at a temperature of 1,270l,285F., and the zinc in the space 72' between the liner 44' and crucible 42' was at approximately the same temperature. A strip of steel having a thickness of 0.007 inch and a width of 6 inches was passed by the deposition nozzle 16 at a rate of 70 feet per minute and was spaced from the nozzle 1 inch. The temperature within the deposition chamber 26 was maintained at room temperature, and the vacuum within the chamber 26 and the vaporization unit 28 was maintained below 20 microns of mercury pressure. With this arrangement, the evaporation rate of metal was approximately 0.36 pounds per minute, and metal was deposited on the strip at approximately 0.005 pounds per foot-minute. The coating that was obtained was uniform and of an average thickness of 600 X inch.

It should be noted that the invention has been described in connection with batch plating processes in which a fixed quantity of metal is supplied to the crucible 42 (42) for vaporization. The invention is, of course, suitable for a continuous plating process in which metal to be vaporized is continuously fed into the crucible, for example by application of a strip, wire or liquid through a suitable opening in the cover 50 (50'). By providing a fixed volume of molten metal for boiling, the vaporization rate may be maintained constant regardless of the overall amount of molten metal available for vaporization. By maintaining the crucible under vacuum in the inside and outside thereof, problems of sealing are obviated and problems of corrosion are avoided in the event that metal for deposition enters into the enclosure 28 outside the crucible.

The invention described above has been set forth in the context of a representative embodiment, which is susceptible to modification. Accordingly, the invention should be taken to be defined by the following claims.

What is claimed is:

1. A method of vaporizing metal, comprising forming a thin shell of molten metal that communicates with and is supplied from a body of molten metal, the level of molten metal in the thin shell being higher than the level of molten metal in the body of molten metal, heating said thin shell of molten metal to vaporize it, maintaining said thin shell under vacuum to provide for the production of metal vapor under vacuum, withdrawing vapor from the top of the thin shell and directing vapor and any entrained droplets therein downwardly toward said body of metal and thence withdrawing upwardly the vapor from a zone positioned below the location from which the vapor and any entrained droplets therein is withdrawn from the top of said thin shell while permitting the entrained droplets to continue to fall downwardly into said body of metal, and directing the metal vapor withdrawn from said zone against a substrate to be coated.

2. A method of vaporizing metal and depositing it on a substrate to form a coating on the substrate, comprising providing under vacuum a thin shell of boiling metal that is of substantially constant volume and which communicates with and is supplied from a supply body of molten metal to produce metal vapor under vacuum at a substantially constant rate, and withdrawing metal vapor from said thin shell and directing said vapor and any entrained droplets therein through an exit path in which the flow of vapor reverses direction to remove said droplets in one direction to return said droplets to said supply body of molten metal and withdraw vapor in an opposite direction to be thence directed against a substrate to be coated.

3. A method according to claim 2, including the step of maintaining the temperature of said supply body of metal at just below the boiling point of said metal.

4. A method of vaporizing metal commprising:

l. heating in a first zone a body of metal under vacuum to vaporize it,

2. directing the vapor and any entrained droplets or particles to a second zone which is a. separated from the first'zone and b. disposed in communicating relationship to a reservoir of molten metal which, in turn, communicates withthe first zone to keep said first zone continuously supplied with molten metal for vaporization,

3. directing the vapor through the second zone along a path of travel which substantially reverses direction to remove any entrained droplets from the vapor so that said droplets may flow to the reservoir and ultimately back to the first zone, and

4. directing the substantially droplet-free vapor from the second zone onto a substrate to be coated. *l i 

2. A method of vaporizing metal and depositing it on a substrate to form a coating on the substrate, comprising providing under vacuum a thin shell of boiling metal that is of substantially constant volume and which communicates with and is supplied from a supply body of molten metal to produce metal vapor under vacuum at a substantially constant rate, and withdrawing metal vapor fRom said thin shell and directing said vapor and any entrained droplets therein through an exit path in which the flow of vapor reverses direction to remove said droplets in one direction to return said droplets to said supply body of molten metal and withdraw vapor in an opposite direction to be thence directed against a substrate to be coated.
 2. directing the vapor and any entrained droplets or particles to a second zone which is a. separated from the first zone and b. disposed in communicating relationship to a reservoir of molten metal which, in turn, communicates with the first zone to keep said first zone continuously supplied with molten metal for vaporization,
 3. directing the vapor through the second zone along a path of travel which substantially reverses direction to remove any entrained droplets from the vapor so that said droplets may flow to the reservoir and ultimately back to the first zone, and
 3. A method according to claim 2, including the step of maintaining the temperature of said supply body of metal at just below the boiling point of said metal.
 4. A method of vaporizing metal commprising:
 4. directing the substantially droplet-free vapor from the second zone onto a substrate to be coated. 